PHASE-EQUILIBRIA AND THERMODYNAMICS OF SOLID-STATE CONTACT REACTIONS IN THE TERNARY TA-GA-AS SYSTEM

The phase equilibria of the ternary Ta-Ga-As system and the binary Ta-Ga and Ta-As systems were determined using X-ray diffraction of samples annealed at 800 degrees C and differential thermal analysis up to 1400 degrees C. In the binary Ta-Ga system, peritectic melting of the compounds TaGa3 (520 degrees C) and Ta5Ga3 (1060 degrees C) was detected and a phase diagram is proposed which explains the conflicting literature data for phases in this system. The ternary isothermal section at 800 degrees C was determined and the two invariant reactions L + TaAs less than or greater than GaAs + TaGa3 at 455 degrees C and L + Ta5Ga3 less than or greater than TaAs + TaGa2 at 908 degrees C were identified. The polythermal liquid + solid equilibria are developed schematically, comprising a total of six invariant reactions plus additional equilibria with the gas phase. Tantalum and the compounds TaGa2, Ta3Ga2, Ta5Ga3 and Ta-3 As are not in equilibrium with GaAs at any temperature. However, TaGa3, TaAs and TaAs2 are stable on GaAs at 1 bar up to 455 degrees C, 1150 degrees C and 950 degrees C respectively. These phases, especially TaAs, may be used to reduce the reactivity at a Ta-GaAs interface. Based on the ternary solid state equilibria the thermodynamic data are deduced and the thermodynamic driving forces of all possible competing reactions at the Ta-GaAs interface are calculated. These data support quantitatively the interpretation of the reactions observed in Ta/GaAs diffusion couples.